SINGLE CRYSTAL CASTING OF HPTR BLADE FOR AL-31FP ENGINE presentation
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SEMINAR REPORT ON SINGLE CRYSTAL CASTING OF HPTR BLADE FOR AL-31FP ENGINE
Dillip Kumar Das Mohapatra
SINGLE CRYSTAL HP TURBINE BLADE
HPTR blades are hollow and of single crystal structure. This enhance creep resistance, thermal fatigue and hence enhances the life of the blade.
the shape of the turbine blade has a larger relative thickness and curvature compare to compressor blade
Turbine blades in a jet engine experience:-
Differential thermal stress due to higher turbine inlet temperature.
Centrifugal tensile stresses due to rotational speed as high as 30000 rpm.
Bending stresses due to equivalent impulse load of the gases.
Hot corrosive and erosive effects due to higher temperature of the combustion products.
Fatigue-Due to repeated cycle of loads the structure may fail in a much less load than specified .
What is single crystal
A single crystal or monocrystal, is a crystalline solid in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries.
Grain boundaries have a lot of significant effects on the mechanical, physical and electrical properties of materials. Therefore, single crystals are demanded in many fields, such as microelectronics and optoelectronics, as well as structural and high temperature materials.
SINGLE CRYSTAL CASTING PROCESS
Generally Green wax (100%) is used as pattern material since it has low ash content, high tensile strength, resistance to oxidation, low shrinkage and substantial weld strength.
The wax pattern are produced by injecting liquid or semi liquid wax at a pressure = 5 kg/cm2 in to a metallic split die and holding for 50sec at a temp of = 75Ã‚Â°C ,after that the die is cooled , opened and the wax pattern is extracted .
Pattern inspection and testing
For single crystal case, cent-percent patterns are checked . After inspection if there is any defect found the pattern is repaired otherwise, sent to moulding floor.
For single crystal casting ceramic core is used. The chief in gradient is Quartz sand=10% + Alumina Powder=4% & TiO2=1%+ electro corrondum=68% with this 16% plasticizer is added which act as a binder and for easy collapsibility 1% graphite power is mixed.
To provide sufficient green strength caster oil 30%+Rectified spirit=70% is added.
The above prepared Core mass is injected at a pressure =30 kg/ cmÃ‚Â²in to the core box and is kept at a temp= 135Ã‚Â°C for12sec.
After core leveling it is send for sintering which is carried out at a temperature not exceeding 50Ã‚Â°C.
it involves cleaning, sizing and coating of cores.
Core is checked by using magnifying glass. If major cracks are there and are not repairable it is rejected. If cracks are minor and repairable it is sent for geometric check and then sent for repairing.
All the patterns are assembled together. And then all the assembly shells are washed in Colophony solution.
In this process there are 8 number of coatings from which one is dip coating
For 1st and 2nd coating Ethyl silicate solution is used. All blade assemblies are dipped twice in slurry for 1st and 2nd coating .For these coatings ,mixing of slurry should be carried out at 350rpm.
For 3rd coating fresh slurry is prepared because if used slurry is again used then exactly how much aluminum powder to be added is difficult to predict.
4th to 8th coating is carried out only by dipping root of the blade
Dewaxing is carried out in auto-clave boiler at a pressure = 8 kg/cm2 and holding it for 20 min.
DRYING OF SHELLS
MELTING AND POURING
Ni-based alloys contain 10-20% Cr, up to 8% Al and Ti, 5-10% Co, and small amounts of B, Zr, and C. Other common additions are Mo, W, Ta and Nb.
Single Crystal castings are produced basically by keeping the molten metal in the mould in the liquid stage itself and allow it to solidify in the desired manner.,but In the conventional castings the molten metal starts solidification as soon as it comes into contact with the mould walls. This is avoided in case of SX castings.
It is a horizontally mounted, cylindrical double walled chamber meant for melting the charge
The mould is kept at high temperatures (1500-1600 C), almost close to the pouring temperature, then solidification is allowed to start at a predetermined place, usually the bottom of the mould.
Advantages of single Crystal Casting:-
No grain boundary at all in any axis i.e by Elimination of grain boundaries transverse to the principal tensile stress axis has reduced grain boundary cavitations and cracking, resulting in greatly enhanced creep ,ductility, microstructures
The preferred <001> crystallographic solidification direction, which coincides with the minimum in Youngâ„¢s modulus and is oriented parallel to the component axis minimizes the thermal stresses developed on engine start-up and shut-down, this has dramatically improved the thermal fatigue resistance of the turbine hot gas path components.
can be cast with thinner walls-meaning less material and less weightâ€and have a higher melting point temperature
There is no parting line so dimension across it would not vary.
Grain boundary cracking.
Grain boundaries intersecting the critical leading edge or trailing edges of an aerofoil, which is undesirable.
The process is normally limited by the size and mass of the casting. The upper limit may be 5kg.
Not economical for batch production.
Nearly all military aircrafts are powered by turbojets or turboprops starting from trainers to latest supersonic fighters. Their efficiency mostly depend on turbine inlet temperature i.e. the HPTR blades should sustain high temperature and thermal stresses. So researchers around the world are trying to develop new materials which have high strength and stability at high temperatures to meet these demands of the turbine designers.
Single crystal blades are able to fulfill the above requirements to some extent. By eliminating grain boundaries, single crystal blades have longer thermal and fatigue life, and are more corrosion resistant, These improvements all contribute to higher efficiencies.
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